malloc and free

The call

int *p = malloc(n * sizeof(int));   // bytes, computed from the element type

malloc returns void* — assignment to int* converts implicitly. Always multiply by sizeof the element type, never hardcode 4.

Always check for failure

malloc returns NULL when it cannot satisfy the request (out of memory, or an absurd size):

int *p = malloc(n * sizeof(int));
if (p == NULL) {
    fprintf(stderr, "out of memory\n");
    return 1;
}

Un-checked malloc is how programs crash later, far from the cause.

Exactly one free

free(p);      // memory returned
p = NULL;     // now p cannot be used-after-free by accident

Free takes the pointer back to the START of the allocation — the pointer you got from malloc, unchanged. Advancing it loses the address free needs:

p++;          // legal to walk the array
free(p);      // WRONG — p no longer points at the allocation start

Keep the original pointer (or free a saved copy).

After free

p still holds the old address but the memory is no longer yours — reading or writing *p is undefined behavior. Setting p = NULL immediately after free is a cheap, effective habit.